1. Field of the Invention
The present invention relates to an air vent apparatus for an auxiliary fuel tank, which can ventilate the interior of the auxiliary fuel tank, in a power unit in which an engine room is defined by an engine support and an engine cover connected to the engine support. The engine room houses an engine mounted on the engine support and an auxiliary fuel tank which temporarily stores fuel to be supplied to a fuel injection valve of the engine and to which extra fuel is returned from the fuel injection valve.
2. Description of the Prior Art
In a conventional of an air vent apparatus for an auxiliary fuel tank, an air vent pipe extending from the auxiliary fuel tank opens into an intake system of an engine (refer to, for example, Japanese Patent Laid-Open No. 295172/1992) or is directly opened into an engine room.
The interior of the auxiliary fuel tank is heated to a high temperature and fuel vapor occurs in the auxiliary fuel tank, in part because the auxiliary fuel tank is adjacent to the engine and is easily exposed to the heat radiated from the engine and in part because extra fuel returning from the side of a fuel injection valve has a high temperature. In the case where the air vent pipe of the auxiliary fuel tank opens into the intake system of the engine, the fuel vapor occurring in the auxiliary fuel tank is mixed with air sucked from the engine room by the engine, whereas in the case where the air vent pipe opens into the engine room, the fuel vapor is sucked into the engine together with the air in the engine room. In either case, the air/fuel ratio of the mixture produced in the combustion chamber of the engine becomes high to some extent and may more or less degrade the exhaust characteristics of the engine.